Add ssh_no_proxy parameter (via @gaserre to address issue #84)

[chqr]

Via @gaserre:

If ssh_no_proxy is specified, do not use a proxy (port forwarding)
when executing the provision and shutdown steps.
The reason to avoid proxying is that the current implementation does not
handle ssh disconnects properly.
The VNC boot command still uses the proxy.

[rdobson]

@chqr - can you elaborate on the ssh disconnect issues you're referring to? Perhaps we could fix that?

[chqr]

@rdobson thanks for the reply! The discussion in #84 has the background on this PR.

The ssh disconnect issues were observed by @gaserre, and were his reason for creating this patch in his fork of packer-builder-xenserver.

My team needs this option for a different reason. In the XenServer cluster where we build our Packer vms, the vms use a different vlan than the Xen hosts. It is impossible to connect to the new vm's IP from the Xen host. It is possible, however, for the Packer process to directly connect to the vm. That's why we need to have a way to disable the ssh proxy.

        [vm vlan]
+--------+    +--------+
| packer | -> | new vm |
+--------+    +--------+
   |
===|===firewall=========
   v
+---------+
| xenhost |
+---------+
        [host vlan]